High frequency amplifiers having a variable gain function have been known, as disclosed, e.g., in Japanese Laid-Open Patent Publication No. H10-261925. The high frequency amplifier disclosed in this publication includes a variable attenuator provided between the input matching circuit and the first stage amplifier FET (field effect transistor). Specifically, the drain of the bypass FET of this variable attenuator is connected to the gate of the first stage amplifier PET. Further, the gate of the bypass FET is connected to a gain control voltage source through a resistance. According to paragraph [0027] of the publication, the gain control voltage source is controlled so as to vary the drain-source resistance of the bypass FET and thereby vary the amount of attenuation produced by the variable attenuator. In this way the gain of the high frequency amplifier can be varied.
It should be noted that paragraph [0022] of the above publication No. H10-261925 mentions that the configuration of the above high frequency amplifier may be applied to multistage amplifiers. The paragraph further mentions that in such multistage amplifiers, the second stage amplifier FET may be connected to the output side of the first stage amplifier FET.
Other prior art includes Japanese Laid-Open Patent Publication Nos. S62-143507, 2002-171101, and H01-255469.
Power amplifiers are often required to have a gain switching function, as can be seen from the fact that the above prior art provides high frequency amplifiers having a variable gain function. For example, a gain switching function is often required of W-CDMA (wideband code division multiple access) power amplifiers. The reason for this is that when an RF transceiver LSI (radio frequency transceiver large scale integration) is connected to the input side of such a power amplifier, if the gain of the power amplifier is high when the output power level (Pout) of the power amplifier is low, the output power of the RF transceiver LSI often need be reduced by an amount corresponding to the gain of the power amplifier.
In that case, the ratio of the signal level to the noise level decreases unless the noise level at the output of the RF-LSI is sufficiently low, resulting in degradation of the signal-to-noise ratio (SN ratio) of the output signal of the RF-LSI. One effective way to avoid this problem is to reduce the gain of the power amplifier at low output power levels, thereby improving the SN ratio. An attenuator may be used to achieve the function of reducing the gain of the power amplifier. The term “attenuator” may be hereinafter abbreviated as “ATT” for convenience of explanation.
FIG. 20 is a diagram showing an exemplary configuration of a two-stage amplifier having an attenuator. More specifically, FIG. 20 is a block diagram showing a specific GaAs-based power amplifier manufactured by a BiFET or HBT process. Referring to FIG. 20, an ATT is provided between the first stage amplifier and the second stage amplifier. A bias circuit is provided for each of the first and second stage amplifiers. A reference voltage generator (Vref-Generator) generates a reference voltage Vref from a voltage Ven, and this reference voltage Vref is applied to the bias circuits.
If an ATT is provided on the input side of the power amplifier (i.e., on the terminal IN side of the first stage amplifier shown in FIG. 20), when the ATT is in its attenuation state (or ATT state), the noise factor (NF) of the power amplifier is degraded by an amount corresponding to the amount of attenuation. That is, a power amplifier in which an attenuator for gain switching is provided between the input matching circuit and the first stage amplifier (as in the high frequency amplifier disclosed in the above Japanese Laid-Open Patent Publication No. H10-261925) may suffer such noise factor degradation. Further, the present inventor has found that the high frequency amplifier disclosed in the above publication is also disadvantageous in that the phase shift in the amplifier significantly changes when its gain is switched between high and low levels, since the gain switching is accomplished by connecting and disconnecting, by use of a switch, a capacitance provided in the input stage to switch the gain.
The noise factor degradation can be avoided by providing an attenuator between stages, as shown in FIG. 20.
However, as a result of intensive study, the inventor has found that two-stage amplifiers having an attenuator between the stages, as shown in FIG. 20, exhibit increased input reflection loss and an increased change in phase shift therein. That is, when the gain of these amplifiers is switched, the input reflection loss may increase significantly or the phase shift may change significantly, depending on the configuration of the interstage attenuator. The present inventor has found a novel technique that avoids such degradation of the high frequency characteristics by employing an interstage attenuator having a specific configuration.
The present invention has been made to solve the above problems. It is, therefore, an object of the present invention to provide a power amplifier having an attenuator between stages wherein the gain of the power amplifier can be switched while preventing degradation of the input reflection loss therein.
Another object of the present invention is to provide a power amplifier having an attenuator between stages wherein the gain of the power amplifier can be switched while suppressing change in phase shift therein.
Other and further objects, features and advantages of the invention will appear more fully from the following description.